The present invention relates generally to the use of thick film technology for forming resistive elements on ceramic substrates for the measurement of strain. More particularly, the invention relates to the use of such technology in ceramic diaphragm pressure gauges.
It is generally known in the art to use resistive strain gauge technology in the measurement of differential pressure by attaching piezoresistive elements to a diaphragm in order to measure the strain in the diaphragm as the diaphragm is deflected by the applied pressure. The term "differential pressure" as used herein shall mean a difference in pressure between the first side and the second side of the diaphragm. The first side has the pressure to be measured applied to it and the second side has a reference pressure applied that may be any pressure commensurate with the design, including a vacuum. The term "piezoresistive element" as used herein shall mean an element that undergoes changes in its electrical resistance properties when subjected to strain. To use piezoresistive elements to measure differential pressure, the elements are incorporated in a bridge circuit for measuring changes in resistance, which are translated into pressure values. The term "bridge" as used herein shall mean an electrical configuration used for balancing voltages, otherwise known as a "Wheatstone Bridge." An example of such a pressure sensor is disclosed in U.S. Pat. No. 4,771,261, in which a flat, ceramic diaphragm member has a face on which thick-film piezoresistive elements are formed. These elements are interconnected with circuitry also formed on the diaphragm. The diaphragm is bonded around its outside edge to a disk-shaped support with the circuitry and piezoresistive elements facing the support. The support includes a central recess to permit deflection of the diaphragm. Holes in the support provide for interconnection of the circuitry printed on the diaphragm with remote circuitry printed on the reverse side of the support.
In general, the piezoresistive elements, as well as the circuitry interconnecting those elements, are "thick-film" images formed on the ceramic substrate using a silk screen printing process. Both the piezoresistive elements and the interconnecting circuitry are applied to the substrate in the form of an ink or paste, comprising a conductor such as a noble metal, a dielectric and an organic adherent. The components are cured at a high temperature after being applied to the substrate, burning away the organic component.
Because the piezoresistive elements are applied using a silk screen printing process, resolution is limited, and the practical lower limit for the width of a resistor line formed in this manner is approximately 0.50 mm. Additionally, the sensitivity of the bridge circuitry used in conjunction with such strain gauges requires a minimum conductivity of the resistive elements, further limiting the minimum width of the resistors.
The resistors generally contain a much lower concentration of noble metal than do the circuit pads they connect. During firing, there is therefore a tendency for the noble metal in the circuit pads to leach into the piezoresistive elements and for some of the constituent parts of the resistive ink to be scavenged by the metal pads, causing undesirable thermal effects, increasing the conductivity of the piezoresistive elements, and thus adversely affecting gauge precision. This problem is aggravated by the minimum width required in the piezoresistive elements as discussed above. As the width of the piezoresistive elements increases for any fixed length, so does the leaching problem. It is desirable to create a piezoresistive element that has a high length to width aspect ratio, the former dimension being greater than the latter.
It is also known to combine the ceramic diaphragm and the support component into a one-piece unit, as disclosed in U.S. Pat. No. 4,898,035 to Yajima et al. That patent discloses an integrally formed ceramic cup-shaped sensor housing incorporating a diaphragm as well as a cylindrical wall. The piezoresistive elements are formed on the inside surface of the diaphragm facing the inside of the cylindrical wall. The elements are formed using a printing process. Known screen printing processes, however, do not permit placing the resistive element in an optimum position close to the corner formed by the diaphragm and the cylindrical walls, where strain created by deflection of the diaphragm is a maximum. Further, the internal diameter of the cup must exceed a minimum size in order to provide sufficient clearance for screen printing the resistors and circuitry on the base. There is therefore a need for an improved process for applying the resistive ink that permits the formation of strain gauge elements within confined spaces.
Still further improvements in the art would be desirable. In particular, there is a need for a piezoresistive element for a strain gauge having increased sensitivity while at the same time having sufficiently high conductivity to permit accurate measurements using a bridge circuit. In addition, there is a need for a method for applying resistive ink to the ceramic substrate capable of forming extremely thin lines having a high aspect ratio, and permitting the formation of such lines in confined spaces such as the inside of a small, cup-shaped ceramic pressure sensor.